In the aeronautical field, safety constitutes one of the fundamental parameters. Having regard to increases in air traffic, aircraft manufacturers and airline companies are imposing ever more ambitious objectives on equipment manufacturers. In the field of cockpit displays, any display of erroneous images is henceforth prohibited.
For many years, liquid-crystal flat screens have been prevalent in the field of displays. They are, inter alia, used to effect the displays of aircraft instrument panels.
Conventionally, a liquid-crystal display, termed LCD, essentially comprises a lighting source and a matrix-like optical modulator. The matrix proper is a pane composed of a stack of various layers. FIG. 1 represents a partial exploded view of an LCD matrix. In this view, the white arrow indicates the direction of propagation of the light through the matrix. The latter comprises in succession:                A first rear polarizer 1 disposed on the lighting source side;        A first glass sheet 2 which comprises the matrix control electronics 3 composed mainly of a horizontal control bus and of a vertical control bus, the control electronics commonly being called “drivers” according to the conventional terminology;        A first support plate 4 for the liquid crystal;        The liquid crystal 5;        A second support plate 6 for the liquid crystal bearing a counter-electrode also called the “backplane” 7;        A matrix network 8 of triples of colored filters. Each triple corresponds to a pixel also known by the term colored “dot” of the image;        A second glass sheet 9;        A second rear polarizer 10 disposed on the observer's side.        
The display operates as follows. The light source is polarized at the rear of the pane by the first polarizer 1. The light passes through the liquid crystal, the colored filters 8 and emerges through the second polarizer 10. The polarization of the light is phase-shifted by 90 degrees when it passes through the liquid crystal whilst quiescent.
There are two chief possible operating modes. In the first mode, the polarization axis of the second polarizer is perpendicular to that of the first polarizer. In this case, the light issuing from the pane, after passing through the liquid crystal, has the same polarization state as the second polarizer and can emerge. This mode is called the “white mode” or else “normally white”. In the second mode, the polarization axis of the second polarizer is parallel to that of the first polarizer. In this case, the light issuing from the pane is polarized at 90 degrees to the polarization axis of the second polarizer and cannot emerge. This mode is called the “black mode” or else “normally black”.
In both cases, following the drive command applied to the liquid crystal, the light passing through the latter will be phase-shifted by it to a greater or lesser extent, and only a fraction of the light passes through the front polarizer as a function of the phase shift generated. Gray shades are thus created on each colored filter. It is thus possible to generate a pixel or a “dot” having a given color either in “normally white” mode or in “normally black” mode.
The first LCD screens used solely a structure termed “twisted nematic” or TN. This structure made it possible to produce LCD cells termed “normally white”. Not driven, the “dots” were luminous.
In the aeronautical field, the dots of colors were organized into quadruples called “quads” and column “drive” or control circuits, mounted interleaved, so-called “Stripe” mode, were used in order to cover the loss of a video link.
These cells used a drive mode called “backplane switching” to control the matrix.
Moreover, the first displays exhibited technological weaknesses. The liquid crystal had a low time constant and the amorphous silicon “MOS” transistors had sizeable current leakages.
Now, avionics graphical images generally use a dark background to improve the contrast of the plots. On the first LCD screens, a fault then created an abnormal luminous zone that the pilot detected immediately. Consequently, the technical characteristics of the first LCD displays readily allowed visual detection of faults in the display and the associated electronics. In conclusion, safety was ensured naturally.
Progress with liquid crystals, with column drivers and with the manufacture of active matrices has allowed the use of a drive mode of control called “fixed backplane”. The viewing angle of the matrices has been increased by introducing new structures and new configurations of matrices. So-called MVA matrices, the acronym standing for “Multi-domain Vertical Alignment” or IPS matrices, the acronym standing for “In Plane Switching”, will be cited by way of example. These new matrices are in “normally black” mode. The non-driven cell is therefore black. This therefore minimizes the effect of faulty pixels which are then predominantly black, contrary to “normally white” TN type matrices whose predominantly luminous defective pixels are abundantly evident.
Of course, these matrices which possess better optical performance are used in the aeronautical field. Unfortunately, these cosmetic or esthetic advantages introduce a complication as regards safety. With these new matrices, a fault creates a dark zone which may seem normal, whereas the useful information has disappeared. Thus, a fault with a video link may cause the loss of the red pixels. This fault makes the red alerts disappear and transforms the yellow and orange alerts into green-colored information. Moreover, faults with the “line drivers” of LCD matrices create frozen images which may have a remanence of the order of a minute and are therefore deemed unacceptable. These events are obviously strictly prohibited in aeronautical applications.